1. Field of the Invention
The invention relates to foamable compositions based on organosilicon compounds, to silicone-containing polyisocyanurate foams with low densities, and to processes for their preparation.
2. Description of the Related Art
Despite the fact that in recent decades there has been no lack of intense research activities into improving the flame retardance properties of polymer foams, it has not yet proven possible to establish strongly flame-retarded PU foams on the market.
One relatively successful approach which has emerged to the production of flame-retarded polyurethane foams is polyisocyanurate chemistry. Production of such foams typically involves a reaction of polyisocyanates with compounds having hydrogen atoms that are reactive toward isocyanate groups, such as polypropylene glycols, with the isocyanate index being at least 180. In such a reaction, in the presence of a trimerization catalyst, formation of the urethane structures is accompanied by formation of isocyanurate structures as well. The resulting polyisocyanurate (PIR) foams are, typically, closed-cell, rigid foams, which among all of the types of polyurethane foam exhibit the best fire properties in respect of fire retardance.
Generally speaking, in the production of rigid polyisocyanurate foams, not only blowing catalysts and gel catalysts, usually amines, but also trimerization catalysts are among the catalysts employed. Additionally, catalyst systems consisting of a mixture of different catalysts are found in the prior art. These rigid PIR foams are typically produced using physical and chemical blowing agents. Physical blowing agents used include, for example, chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrocarbons, and also liquid carbon dioxide, while chemical blowing agents used are principally water and carboxylic acids.
Despite the fact that the rigid PIR foams already have relatively good fire properties, there is still a great need for improvement, since high levels of added flame retardants are needed in order to obtain optimized fire retardance. Such flame retardants adversely affect the mechanical properties of the resulting foam, and, furthermore, are not always toxicologically acceptable.
It would therefore be desirable to have a rigid foam which is characterized by improved fire properties, has good mechanical properties coupled with low foam densities, and can be used without the addition of toxic flame retardants.
One such route to flame-retarded PU foams is taken in the silicone-polyurethane foams. In such foams, the highly combustible polyol component that is used in standard PU foams is replaced by poorly combustible, OH-terminated siloxanes. Through the use of silicone-polyurethane copolymers, i.e., of polysiloxanes, which also contain polyurethane units and/or urea units, it is possible to develop fireproof foam materials of this kind which have new combinations of properties that are tailored precisely to the particular application.
Reference for this may be made, for example, to WO 2009130194A1, which describes the production of rigid silicone-PIR foams starting from hyperbranched alkylhydroxy-terminated silicone oils and in what is called the “one-shot process”.
What is disadvantageous about such processes, however, is the fact that foam structures are no longer completely closed-cell at foam densities below 70 kg·m−3. This tendency becomes more and more dominating with decreasing foam densities, and so these foams lose their insulating effect.